Nickel hydroxide is an important material in the manufacture of positive nickel electrodes for alkaline batteries and industrial applications. Essentially, all known processes for making nickel hydroxide are based exclusively on caustic precipitation of nickel hydroxide from nickel salt solutions. For example, typical nickel salt solutions for forming nickel hydroxide include nickel sulfate, nickel chloride or nickel nitrate. Unfortunately, methods of production that rely upon caustic precipitation of nickel salts involve a number of operating steps and produce environmentally unacceptable effluents.
The direct conversion of Ni powder into nickel hydroxide was first described in Australian Pat. No. 626,648 ('648). The process of the '648 patent disclosed the use of ammonia and oxygen under pressure to form nickel hydroxide from nickel powder in aqueous solutions. However, the '648 patent states that conversion of Ni powders into nickel hydroxide in ammonium hydroxide solutions can be achieved, at practical rates, only at elevated temperatures and oxygen partial pressures. At lower temperatures and oxygen partial pressures, the rate of Ni conversion is impractically slow. For example, only about 0.1% Ni dissolved from nickel powder in a 10 molar NH.sub.3 solution at 55.degree. C. and 2 atmospheres oxygen partial pressure during 4 hours of batch reaction.
It was later disclosed in EPO Pat. Publication No. 575,093 ('093) that when using nitric acid as an oxidant of nickel powder, atmospheric operations may be used to produce nickel hydroxide. However, the problem with the process of the '093 publication is that nitrate impurity levels exceed customer requirements for some applications. The use of nitric acid is believed to cause entrainment of nitrate in the nickel hydroxide.
It is an object of this invention to provide a method for producing nickel hydroxide from nickel powder at atmospheric pressure conditions.
It is a further object of this invention to provide a method of reducing nitrate impurities.
It is a further object of this invention to control crystallinity of nickel hydroxide produced from nickel powder.